Fisheries biology of Pennahia argentata in the north-central Taiwan Strait and in Ariake
Sound Japan was examined based on samples collected from two different area and
objective studies. In the north-central Taiwan Strait between March-November
2006, a total of 994 specimens were collected from single trawler fishery. Fish
size ranged from 58 to 267 mm in length and 5 to 410 g in total wet weight. Ages
was determined or back-calculated between 0 to 5 years. The minimum size at
first maturity for female 119 mm and 115 mm for male. Compare with previous studies, P. argentata has decreased in recent
decades, indicating that the populations were younger, smaller and earlier in
sexual maturity. Meanwhile, Mortality coefficients were higher to be estimated
total mortality (Z) was 1.7410,
nature mortality (M) was 0.8690,
fishing mortality (F) was 0.8720, and
exploitation ratio (E) was 0.5009
indicated that fish have been overfishing. In Ariake Sound between September
2001 and Augustus 2002, a total of 421 females and between 14 July – 4 Augustus
2004, a total of 830 mature female were collected. The reproductive biology of P. argentata was studied by measurement
of gonadosomatic indices (GSI) and histological examination of ovaries. Ovaries
were classified into five maturity stages: (i) immature/resting; (ii)
developing; (iii) maturing; (iv) ripe/spawning; (v) spent. The spawning season
lasts from April to September with peak activity from May to August. Age at
sexual maturity was assumed to be 1 year. The diel-changes in the proportion of
females with postovulatory follicles and oocytes at maturation stage, most
females spawned between 19:00 and 21:30 h.

Pennahia argentata (Houttuyn, 1782) is a demersal fish belonging to the
Sciaenidae family and inhabits sandy or
muddy bottoms in the coastal inlets to a depth of 40~100 m. Distribution is from Tohoku, Japan to the Bohai
Sea, the East China Sea, and the Indo-Pacific
(Yamada, 2000). P.
argentatahave two common name are silver croaker or white croaker.

P. argentata
is a part of the most common commercial fishery resources shared by Fujian and
Taiwan provinces in the Taiwan Strait. (Fisheries Research Institute of Fujian, 1986; Island Joint Investigation
Team, 1993). Meanwhile, P. argentata is one of the major species
found in demersal fish assemblages off the coasts of Japan and is an
important part of Japan’s commercial fisheries (Taki, 2000).

The objectives of this work were to: (i) estimate
the age, growth, mortality of P.
argentata in the north-central Taiwan Strait;
(ii) the histology of ovarian tissue used to describe maturation, spawning
season, age at sexual maturity and diel-reproductive periodicity in Ariake
Sound.

Materials and Methods

Sampling
area

In
north-central Taiwan Strait

A total of P. argentata994 specimens
were randomly sampled monthly in 80~120 individuals from 10 stations from a
trawler in March-November 2006 (Figure 1). The trawl net was 60 m in length and
125 m in entrance perimeter, with a 25 mm mesh size.

In
Ariake Sound

Between September 2001 and Augustus 2002, a total of 421
females were collected from commercial bottom trawling and gill nets to describe maturation,
spawning season and age at sexual maturity. Between 14 July – 4 Augustus 2004, a total of 830 mature females were collected
from commercial
bottom trawling to describe diel-spawning periodicity. All speciemens were collected in
the middle part of Ariake Sound (Figure 2).

Methods
Analysis

Age determination

The Fraser-Lee procedure (Lee, 1920) was used to
back-calculate Standard Length (SL) associated with each annulus for each
age-group: .

Growth

Length-weight relationship
were estimated by the allometric equation (Sparre and Venema, 1992): .

Gonadosomatic indices were
calculated to “assess maturity”. Maturity stages of ovaries were determined on
the basis of the most advanced oocytes, and the occurrence of atretic oocytes
and/or postovulatory follicles. The Spawning season was established on the
basis of the mean monthly variation of GSI and ovarian maturity stages. Age at
sexual maturity was estimated by the proportion of mature fish in each year
class.

Diel-spawning
periodicity

Diel-spawning periodicity
was estimated by calculating the proportion of individuals with hydrated
oocytes and postovulatory follicles in histological preparations of ovaries
from all collected females.

Results

Size and age structure

P. ArgentataStandard Length (SL) ranged from 58 to 267 mm (Figure
3). Age of P. Argentata were determined or
back-calculated between 0 and 5 years (Figure 4).

Growth

Relationships between SL and otolith radius of the
species was described as P. Argentata:

Total wet weight-standard
length relationships of P. Argentata was calculated.

The von Bertalanffy
growth (VBGF) equations were estimated to beThe VBGF () of P. Argentata is the non-infection point curves which increase with
age and tend to the asymptotic value. The VBGF () of P. Argentatais ‘S’ type curves and have points of inflection (Figure 5), indicating that the total wet weight increase with age
experience by the Low – fast – Low, and then tend to asymptotic total wet
weight.

Population structure

Population structures have changed in recent decades(Table 1). P. argentata mean SL decreased from
140.6 to130.7
mm, and mean age from 1.42 to 0.99 years.

Table 1. Population
structure, standard length, total wet weight and age changes

Standard length composition (mm)

Total wet weight composition (g)

Age composition (years)

Species

Sampling years

Range

Dominant group

Mean

Range

Dominant group

Mean

Range

Dominant group

Mean

Authors

Pennahia argentata

1982

67-258

121-150

140.6

8-395

51-90

82.4

0-5

1

1.42

Zhang (1987)

1994

65-216

111-150

132.2

7.5-242

41-90

64.7

0-3

1

1.09

Lu et al., (1999a)

2006

58-267

111-150

130.7

5-410

45-90

61.4

0-5

1.0

0.99

Current study

Mortality

Survival (S)
was broadly estimated using two methods fromHeincke (1913) S1
was 0.1655 and Robson and Chapman (1961) S2 was 0.1858; we thereforeadopted the average
value of the two S to calculate totalmortality (Z). Mortality coefficients (total mortality[Z] was 1.7410, nature mortality [M] was 0.8690 and fishing mortality [F] was 0.8720) andexploitation
ratio (E) was 0.5009. Thehigh F and E indicated that both species suffered great fishingpressure in this sea
area.

Maturity stages

Oocyte development was classified into seven stages:
(1) peri-nucleolus, nucleoli distributed around the inner margin of the nuclear
membrane (Figure. 6a); (2) yolk vesicle, a large number of oil droplets present
within the cytoplasm (Figure. 6b); (3) primary yolk, yolk globules first appear
in the cytoplasm (Figure. 6c); (4) secondary yolk, yolk globules increased in
size and number and occupied most of the cytoplasm (Figure. 6d); (5) tertiary
yolk, yolk globules further increased in size and number, many oil droplets
fused with each other (Figure. 6e); (6) migratory nucleus, the nucleus migrated
from the center to a pole, and more oil droplets had fused (Figure. 6f); (7)
maturation, a single yolk mass originated from the yolk globules (Figure. 6g). Newly
formed postovulatory follicles (Figure. 6h) were found in many specimens with
oocytes at yolk and mature stages. Degenerated oocytes were observed in
specimens with oocytes at peri-nucleolus stage in September.

The GSI of females showed a distinct seasonal trend
(Figure. 7). The GSI increased rapidly in May and declined sharply in
September.

The mean frequency distribution of ovarian maturity
stages is shown in Figure. 8 and also has distinct seasonal trend. Females at
stage II, which commenced ovarian maturation, occurred in March. Females at
stage III started to occur in April, and females at stage IV, having oocytes at
maturation stage that were ready to be ovulated, were observed during May-August.
Specimens with spent ovaries (stage V) were first observed in September. The
spawning season lasts from April to September with peak activity from May to
August.

Age at sexual maturity

Females at maturity stages II-V were estimated to be
mature. The minimum size of female specimens collected from May to August 2002
was 139 mm in total length (). All specimens had
mature gonads. In Ariake Sound, the total length at 1 year for white croaker
approximates 160 mm in females (Higuchi et al., 2003). Accordingly, age at
sexual maturity for females we collected was estimated to be 1 year.

Diel-reproductive
periodicity

Females with hydrated oocytes occurred most
frequently between 15:30 and 21:30 h throughout the sampling days, then
decreased sharply after 22:00 h (Figure. 9). The occurrence of females with
postovulatory follicles were most frequent during 19:00 – 21:30 h, just after
the peak of spawning and decreased sharply afterward (Figure. 10). These data
indicate that most females spawned between 19:00 and 21:00 h.

Discussion

Growth and population
structure

The growth performance index ()
was used to compare the growth rate in different areas. For P. argentata, from Taiwan Strait was lower than in the East
China Sea (Hu and Qian, 1989) and the northern South China Sea (Chen et al.,
2010) but higher than in Japan Ariake Sound (Higuchi et al., 2003) and Omura
Bay (Yamaguchi et al., 2004). Specific ecological factors, such as population
density, environment temperature, and prey density, may cause growth parameters
for P. argentata to vary in different
sea areas.

Population structures have
changed in recent decades. P.
argentata mean SL decreased
from 140.6 to 130.7 mm; mean age from 1.42 to 0.99 years. P. argentata was smaller in size and younger in age.

Over-exploitation:
overfishing

The mortality coefficients
(Z, F), especially F, increased in recent decades, indicating this
species suffered from increased fishing pressure. The exploitation rate (E)
in 2006 is 0.5156 reached or exceeded an optimally exploited value (E =
0.5: Gulland, 1971), indicating that the stocks had been overfishing.

Spawning season and age
at sexual maturity

In the present study, all specimen was estimated on
the basis of GSI and histological examination of ovaries. The spawning season
lasts from April to September and is concentrated from May to August in Ariake
Sound. Age at sexual maturity for females was estimated to be 1 year. Based on the
diel changes in proportion of females with hydrated oocytes and postovulatory
follicles, white croaker is suggested to spawn most frequently between 19:00
and 21:30 h.

In previous study of ovarian GSI, the spawning
season of P. argentata was shown to
lasts from March to July with peak activity from May to June in the northern
part of the East China Sea and the Yellow Sea (Saishu et al., 1954) and from May to September with peak activity from
July to August in the middle and southern parts of the East China Sea (Saishu
et al., 1954). There are no differences in the spawning season between Osaka
Bay (Yoshida et al., 1997) and Ariake Sound based on the mean monthly GSI.

It was estimated from GSI that female P. argentata reach sexual maturity at 1
year in Osaka Bay (Yoshida et al., 1997) and 2 years in the East China Sea
(Saishu et al., 1954) and the Yellow Sea (Saishu et al., 1954). These
variations in spawning season and age at sexual
maturity of females may be an adaptation to local environmental conditions for
each population.

Diel-reproductive
periodicity

The
diel period of peak spawning is often uniform within a
taxonomic family (Ferraro, 1980). It
was different between white croaker spawn between 19:00 and 21:30 h (Current
study) late compared to Nibea albiflora in Ariake Sound spawn between
15:00 – 19:00 h (Takita, 1974).